Do loooots and lots of oil paintings.
Craig was a master painter way before Photoshop and Painter where even a workable medium to use, but becuase of his phenominal understanding of paint and color, he’s able to get those great effects. Check out his actual work and not just the “Sketches” of his in order to get a true scope of what he’s capable of. Your not going to be able to jump right to that level instantly.
Try out this tutorial to get a basic Idea of one of the ways he works,
http://www.gfxartist.com/features/tutorials/7731
But ultimatly, just study painting, Use his sketches as a great guide about where to add more detail, less detail, and most of all color. Color therory is what realy captivates most of his paintings, so study it. Also, unless your working on stuff for games, be prepared to study up on alot of lighting and rendering. I recomend Jerremy Birn’s book. It might be a bit out dated at this point in time, but I’ve found that its the easyest book to digest if you are just getting into this.